Over the past decade tremendous efforts have been made optimizing the synthesis of semiconductor nanocrystal colloids. These ‘quantum dot’ materials, exhibiting 3D quantum confinement, are highly desired for their size-tunable optical properties. Synthetic routes utilizing organometallic precursors enables production of nanocrystalline particles with nearly monodisperse size dispersions as disclosed in (1). The resulting narrow emission bandwidths and luminescence efficiencies that are achieved through these controlled syntheses as disclosed in (2, 3, 4, 5) are required for applications such as optoelectronic devices (6–10) and biological fluorescence labeling (11–13). The explosive growth of the telecommunications industry has fueled the pursuit of luminescent quantum dots that emit at key NIR wavelengths of 1.3 and 1.55 μm. The vast majority of colloidal quantum dot research has focused on II–VI and III–V materials and includes substantial efforts in the development and exploration of InAs nanocrystalline materials (14, 15). Despite having a bulk band gap of 0.36 eV thus spanning the desired NIR energy region, InAs is a less than desirable material with which to work. Arsenic is highly toxic, and the synthetic routes to prepare the reaction precursors are extremely dangerous. Alternatively, semiconductors of group IV–VI materials, notably PbS with a bulk band gap of 0.41 eV, offers comparable size tunability and can be produced with inexpensive and effectively safer synthetic precursors.
Lead chalcogenide quantum dots are highly desirable as well for the strong-confinement limit offered by these small-band gap materials. Given that most II–VI and III–V semiconductor materials have large hole masses, strong quantum confinement is difficult to achieve with them. For example, as discussed by Wise (16). InSb has an exciton Bohr radius of 54 nm but the Bohr radius of the hole is only 2 nm, thus preventing strong confinement of the hole. In comparison, the electron and hole Bohr radii of the lead chalcogenides are an order of magnitude larger and imply large confinement energies. In the limit of strong confinement the third-order nonlinear optical response of PbS nanocrystals is expected to be huge, 30 times that of GaAs and 1000 times that of CdSe materials, thus rendering PbS highly desirable for photonic and optical switching device applications.
The pursuit of synthetic routes for the production of lead chalcogenide quantum dots is not a recent endeavor. These methods include solution phase, (17, 18) gas phase, (19) and solid-statei syntheses (20) as well as polymer films, (21) and glass host fabrication (22–24). The limitation of solution phase and gas phase syntheses is the inability to achieve tunable particle sizes. Conversely, the preparation of PbS and PbSe QDs in glass matrices has been extremely successful. These size tunable materials are narrowly dispersed, as evidenced by the well-resolved features in the absorption spectra.
A major drawback shared by all of the aforementioned methods is the inability to readily isolate the materials thus preventing incorporation into alternative media for post-synthetic applications such as device fabrication. This limitation is lifted when working with colloidal quantum dots prepared by the solution phase synthesis utilizing organometallic percursors such as that developed for the production of II–VI materials (2).